The present invention relates to a flow measuring device and, more particularly, to an apparatus for measuring the flow rate of a gas.
An air flow measuring apparatus having a heat generating resistance body such as a platinum wire mounted in a path of air flow by means of an attachment is known from the specification of U.S. Pat. No. 3,824,966 issued to Schneider et al. on July 23, 1974. This air flow measuring apparatus suffers, when used as an air flow measuring apparatus for measuring the intake air flow rate in an internal combustion engine, that the wire is burnt out or deteriorated due to the reverse flow of gas in the event of a back fire and, accordingly, has a poor durability. In addition, the adjustment of the tension of the wire is extremely difficult. Namely, if the tension is excessively large, the wire becomes more liable to be broken. To the contrary, a too low tension inconveniently permits the wire to be vibrated to cause a degradation of the precision of measurement.
The U.S. Pat. No. 3,559,482 granted on Baker et al on Feb. 2, 1971 proposes to dispose an air flow measuring apparatus in a bypass passage between an opening disposed at a portion of the air intake passage downstream from a venturi and an opening formed at a portion of the same upstream from the venturi. This air flow measuring apparatus is superior in that it can shelter from the reversing flow of gas in the event of back fire and that the installation of the air flow measuring apparatus is comparatively easy. In addition, the size of the apparatus can be made considerably small.
These known air flow measuring apparatus, however, involve a common disadvantage that the detection or measuring characteristic is determined by the construction of the air passage or the bypass passage and the calibration characteristic cannot be altered easily.
For instance, when these known air flow measuring apparatus are applied to engines having different capacities, it is necessary to modify the shape and construction of the air passage or the bypass passage and, in addition, to partially modify the arrangement of the platinum wire and the circuit arrangement.